Optical microresonators, using which light can be enclosed in an ultra-small space for a long time, are known in the prior art. These microresonators can be used as microlasers, for example, in particular micro-cup lasers, and are producible in large piece counts and high component density on a chip basis. These structures can be used, inter alia, for the employment as a laser light source in chip-based photonic systems and as a central detection element in the meaning of a biosensor.
Conventional microresonators typically consist of a substrate, a pedestal, and the actual resonator. The resonator, for example, in cup shape, is connected in this case via the pedestal to the substrate, which is typically a silicon wafer and is used as a carrier. The pedestal always consists in this case of the same material as the substrate.
Micro-cup resonators are typically manufactured according to earlier descriptions by a four-step production method. In the first step, a silicon wafer is coated with a polymer photoresist by means of spin coating, wherein the polymer photoresist forms the resonator material. In this polymer, disks having a diameter in the two-figure to three-figure micrometer range are defined in the second step in the meaning of a positive photoresist by means of photolithographic methods. After a suitable (wet) chemical development of the exposed regions, polymer disks on silicon are obtained. In the third step, these disks are isotropically undercut using xenon difluoride (XeF2), so that the edge regions of the polymer disks are exposed and are connected via the pedestal made of silicon to the silicon wafer. Since the edge regions of the polymer disks are exposed, low-loss light guiding is enabled inside the disks. In the final, fourth process step, the entire structure is heated above the glass transition temperature of the polymer. A reduction of disadvantageous surface roughness in the polymer is achieved by this thermal reflow process. Furthermore, the characteristic cup shape of the cup resonators forms as a result of the reduction of the surface energy. The resonator surface is then generally free of defects and enables optical quality factors greater than 107, which are typically limited by intrinsic properties of the polymer.
The preceding production method has disadvantages in particular with regard to the further use of the microresonators for biosensors, however. The use of XeF2 restricts the production method of the microresonators to silicon as the substrate material, whereby the possible uses of the resonators as biosensors are substantially restricted. In this area of application, for example, transparent (for example, for microscopic applications) and/or mechanically flexible substrate materials would be desirable. In addition, the XeF2 etching procedure is a high-vacuum process, which is very time-consuming (infeed and outfeed of the chips, generation of a stable vacuum, etc.) and additionally presumes a costly infrastructure. In addition, even large XeF2 etching facilities are very restricted in their throughput, since the substrates to be etched cannot exceed a specific maximum size.
XeF2 itself is a very costly and highly corrosive chemical. If the optimum vacuum parameters are not precisely maintained during the etching procedure or if atmospheric contaminants and/or residual moisture are present in the etching chamber, interfering organic reaction byproducts form on the resonator surface during the etching procedure. These are disadvantageous in the further use of the microresonators since, for example, subsequent immobilization of the acceptor molecules, which are absolutely required for biosensors, on the resonator surface can no longer be reproducibly applied. Such changes may be directly detected, for example, by way of a change of the water contact angle on the polymer surface.
The etching using XeF2 is therefore not feasible for mass production, which is efficient in time and costs and is reproducible. However, the undercutting of the disks is absolutely necessary for the production of the resonators, since the functionality of the component as an optical element is first enabled in this way. In addition, it is a further disadvantage that it was previously not possible by way of the XeF2 etching procedure to set the pedestal height independently of the pedestal diameter or to create vertical sequences of pedestals and resonators.